Method of making fuel and fertile elements for nuclear-reactor cores

ABSTRACT

Fissionable-fuel or fertile elements for the core of a nuclear reactor, especially a reactor of the gas-cooled type, wherein the fuel or fertile material is introduced into graphite shells in the form of coated particles and a pyrolytically decomposable carbon-containing or silane gas is passed upwardly through the shell while the mass therein is heated by displacing an induction-furnace core downwardly around the graphite shell thereby bonding the coated particles together with pyrolytic carbon, silicon carbide or zirconium carbide being formed in situ to yield a coherent mass with a porosity of, say, 10 to 25 percent. The gas is preferably a mixture of one or more inert gases (nitrogen, argon or helium) with one or more hydrocarbons or carbon-containing gases (e.g., methane, acetylene, benzene, or silanes).

[ Sept. 3, 1974 METHOD OF MAKING FUEL AND FERTILE ELEMENTS FORNUCLEAR-REACTOR CORES [75] Inventors: Ernii Gyarmati; Hubertus Nickel,

both of Julich, Germany [73] Assignee: Kernlorschungsanlage JulichGesellschaft mit beschrankter, Haftung, Julich, Germany [22] Filed: May3, 1972 [21] Appl. No.: 250,032

Related US. Application Data [62] Division of Ser. No. 49,056, June 23,1970,

abandoned.

[30] Foreign Application Priority Data June 27, 1969 Germany 1932567[52] US. Cl 176/68, 176/67, 176/91 R, 176/91 SP, 264/05 [51] Int. Cl.G2lc 3/70, G21c 3/26, G2lc 21/02 [58] Field of Search 176/44, :45, 46,47, 67, 176/59, 68, 91, 91 SP; 264/05 [56] References Cited UNITEDSTATES PATENTS 3,287,910 11/1966 Silverstein 176/45 3,368,946 2/1968Jenssen 176/59 3,480,702 11/1969 Ford et al. 3,669,832 6/1972 Boettcher176/91 R Primary Examiner-Leland A. Sebastian Assistant Examiner-RogerS. Gaither Attorney, Agent, or Firm-Karl F. Ross; Herbert Dubno [5 7]ABSTRACT Fissionable-fuel or fertile elements for the core of a nuclearreactor, especially a reactor of the gas-cooled type, wherein the fuelor fertile material is introduced into graphite shells in the form ofcoated particles and a pyrolytically decomposable carbon-containing orsilane gas is passed upwardly through the shell while the mass thereinis heated by displacing an inductionfurnace core downwardly around thegraphite shell thereby bonding the coated particles together withpyrolytic carbon, silicon carbide or zirconium carbide being formed insitu to yield a coherent mass with a porosity of, say, 10 to 25 percent.The gas is preferably a mixture of one or more inert gases (nitrogen,argon or helium) with one or more hydrocarbons or carbon-containinggases (e.g., methane, acetylene, benzene, or silanes).

3 Claims, 3 Drawing Figures METHOD OF MAKING FUEL AND FERTILE ELEMENTSFOR NUCLEAR-REACTOR CORES C ROSS-REFERENCE TO RELATED APPLICATION Thisapplication is a division of Ser. No. 49,056 filed June 23, I970 nowabandoned.

FIELD OF THE INVENTION Our present invention relates to fissionable-fuelor fertile elements for use in the core of a nuclear reactor; moreparticularly the invention relates to the operation of nuclear reactorswith particulate fuels and fertile materials.

BACKGROUND OF THE INVENTION It has been proposed heretofore to operatenuclear reactor cores with fissionable-fuel elements of thecoated-particle type and to operate breeder reactors with fuel elementsof this character and/or coated-particle fertile materials.

In coated-particle technology the active material in the reactions ofthe core, e. g., the neutron-emitting fissionable-fuel or theneutron-irradiated fertile material, is encased in substantially fluidimpenetrable sheathsor coating of a ceramic or a material, e.g.,pyrolytically deposited graphite or carbon, having refractoryproperties. Ceramic coatings of silicon carbide, zirconium carbide,niobium carbide, etc. and refractory oxides such as beryllia, alumina,magnesia and zirconia are typical as noted in the JOURNAL OF NUCLEAR MA-TERIALS, Vol. II, pages 1 to 31 (1964). See also commonly ownedapplications Ser. No. 731,791, filed May 24, 1968 (Now US. Pat. No.3,565,762) and Ser. No. 888,819 filed Dec. 29, 1969 (now abandoned).

The particles may be made with coatings as described in this publicationand by the methods set forth therein although it is noted that any otherconventional method of forming the particles may be employed inaccordance with the principles of this invention.

The coated particles generally have a particle size of 400 to 1,000 andmay be spherical or somewhat irregular in cross section. Although theabove-cited publication refers only to nuclear fuels, it will beunderstood that fertile materials for use in nuclear reactors can besimilarly coated and that coated particles of this type are alsoincluded in the scope of the invention although reference may be madefrom time to time to fuel particles as illustrated.

It has been proposed heretofore to embed coated particles of theaforedescribed type in a body to form fertile elements or fuel elementsadapted to be positioned in the core of the nuclear reactor as thereaction-sustaining or breeding material, the binder mixture consistingessentially of graphite particles and synthetic resin which can be curedat a temperature of 180C to fix the coated particles in place. Elementsof this type contain fuel or fertile material and may be furtherenclosed in a graphite receptacle or may be sheathed in graphite byconventional techniques. Such systems have, however, relatively lowdensities in terms of the quantity of active material per unit volumeand do not always afford effective heat transfer between the interior ofthe element and the coolant.

With water-cooled and liquid-metal-cooled reactor cores, therefore, ithas been suggested to place the coated particles in a loosely piledstate within a metallic shell facilitating heat transfer between thecontents of the shell and the externally disposed coolant, butnevertheless failing to improve the heat-transfer efficiency betweencentral portions of the contents of the shell and the shell wall. Thusthe transfer of heat from the interior of the fuel or breeder element isfraught with difficulties which have not been successfully overcomeheretofore.

OBJECTS OF THE INVENTION It is, therefore, the principal object of thepresent invention to provide a fuel or fertile element in which theaforementioned disadvantages are obviated and the resulting element canbe used with effective cooling in gas-cooled or liquid-cooled nuclearreactors.

It is a further object of our invention to provide lowcost fuel orfertile elements of substantially identical characteristics permittingcooling of the inner regions of the fuel or fertile elements therebyproduced.

Another object of the invention is to provide an improved method ofoperating a nuclear reactor in which the problem of cooling the fuel andfertile elements is reduced or eliminated.

Still another object of our invention is the provision of improved fueland fertile elements for use in nuclear reactor cores, in which coolingof the active materials is facilitated.

SUMMARY OF THE INVENTION These objects and others which will becomeapparent hereinafter, are attained, in accordance with the presentinvention, with a system whereby the elements constitute graphitecasings in which the coated nuclearfuel and fertile particles aresintered and bonded to a coherent condition, the resulting massremaining limitedly porous to pennit the circulation of a cooling fluidnot only around the casings of the fuel or fertile element, but throughthe interior thereof to obtain the handling and regulating resultscharacteristic of the use of individual fuel and fertile elements, withimproved cooling.

According to an important feature of the invention, the coated-particlemass within the graphite or other shell is bonded into a coherent bodywhich, in turn, may be bonded to the inner wall of the shell and has aporosity of the order of 10 to 25 percent (i.e., the pore volume is 10to 25 percent of the volume of the parti cles mass) by a progressivesintering or fusion technique whereby a pyrolytically decomposable gasadapted to form binder particles in situ is passed through the looseparticle mass in one direction while heating is progressively advancealong the mass in the other direction so the pyrolytically precipitatedbinder substance first formed and thereafter fused to the coated fuel orfertile particles while the gas continues to traverse the mass to leavethe system porous but coherent. The pyrolytic deposit is preferably of amaterial identical with or equivalent to that of the coating of theparticles i.e. pyrolitic carbon where the particle coating is of thismaterial or silicon carbide where the coating is SiC or zirconiumcarbide.

According to the method of the invention, which affords a simple andeconomical production for fuel and fissionable elements, the fissionablefuel or fertile material is formed into coated particles in which thecoating is pyrolytic carbon (graphite) and/or any of the ceramiccarbides or oxides disclosed in the aforementioned article, theparticles having a particle size of 400 to l,000p.. The coatings may becarbides such as silicon carbide, zirconium carbide, beryllium carbide,niobium carbide, etc., or oxides such as beryllium oxide, aluminumoxide, magnesium oxide or zirconium oxide. The coated particles in anamount of the order of hundreds to tens of thousands, with a preferredparticle size of about 600p. are introduced into an upright graphiteshell which is preferably of cylindrical configuration and is formedwith a perforated bottom through which the pyrolytically decomposablegas is admitted.

The progressive heating of the mass is effected by an induction-furnacearrangement, according to the invention, the coil of which surrounds theshell and is axially displaceable relative thereto to advance theheating zone from top to bottom. The heating process which requiresl,200 to 2,l()C is designed to form bridges between the coated particlewith carbon precipitated from the gas by pyrolysis of a gaseoushydrocarbon such as methane (or another low-molecular weight alkanehaving I to 4 carbon atoms), acetylene and homologues, benzene and itshomologues and or with silicon carbide precipitated by breakdown ofpyrolyzable silanes, such as the chloromethylsilanes. The gas mayinclude an inert carrier gas such as argon, helium or nitrogen.

The induction coil of the induction furnace is positioned, according tothe invention, in axial alignment with the graphite sleeve andthereabove and can be shifted downwardly along the graphite sleeve toprogressively displace the pyrolysis zone in the same direction. Thegaseous reaction products are driven by the pressure of the gas frombelow and the subsequent heating of the fused region to leave smallinterconnected interstices constituting the pores of the mass.

Furthermore, the sleeve may be provided with a multiplicity ofsmall-diameter bores along the walls thereof to facilitate passage ofthe gaseous reaction products from the mass. The bores, both at the baseof the sleeve and in the walls, may have diameters slightly less thanthe minimum diameter of the coated particles, e.g., up to 400,u.. Thefuel and fertile elements, according to the invention, manufactured asabove and introduced into the nuclear reactor, manifest a significantlymore effective cooling throughout the fuel or fertile element when, forexample, the reactor is cooled with helium. Overheating of the interiorof the elements cannot occur. Furthermore, the coherent porous masses ofthe fuel or fertile elements are found to be structurally stable evenwith working temperatures of I,OOOC and above over relatively longoperating periods. Another advantage in the system is that thepyrolytically deposited carbon or SiC bridges between the particlesimpart a thermal conductivity to these bridges which approaches that ofthe coatings themselves, the bonding particles and the coated-particlesheaths having practically identical coefficients of thermal expansionso that the mass does not break down as a result of temperaturevariations.

When the connecting bridges between the fuel and breeder particles areto be formed by thermal decomposition or pyrolysis ofchloromethylsilanes, they consist in major part of silicon carbide and,in order to insure further the continuity of the coefficient of thermalexpansion, it is advisable to employ coated particles in which thesheath consists of silicon carbide. The resistance of the system tocorrosion by moisture is reduced. Moreover, the systems using siliconcarbide and zirconium carbide have been found to be compatible, bothwith respect to thermal conductivity and as far as the coefficient ofthermal expansion is concerned. Interestingly enough, the process of thepresent invention can also be applied to uncoated fuel and fertileparticles which, accordingly, can be bonded together in a graphitesheath.

DESCRIPTION OF THE DRAWING The above and other objects, features andadvantages of the present invention will become more readily apparentfrom the following description, reference being made to the accompanyingdrawing in which:

FIG. 1 is a diagram representing the invention and showing the graphitesleeve or casing in diagrammatic form;

FIG. 2 is a diagram illustrating the formation of bridges between theparticles; and

FIG. 3 diagrammatically illustrates a reactor core provided with fuelelements in accordance with the invention.

SPECIFIC DESCRIPTION In FIG. 1, we show an apparatus for makingfissionable-fuel or breeder (fertile) elements for use in a nuclearreactor in accordance with the invention. The apparatus includes aconventional induction furnace wherein, however, the induction coil 10,which is provided with the usual power supplyv 11, is mounted for axialmovement relative to a support 12 by, for example, a nut arrangement 13engaging a vertical leadscrew 14.

The coil 10 can thus be displaced as represented by the arrow 15relative to a graphite shell 16 mounted on the support 12. The shell orcasing 16 is here shown to be cylindrical but may be of any appropriateconfiguration, provided it is elongated to permit the reaction zone tobe advanced counter to the gas flow as noted earlier. The bottom 17 ofthe shell 16 is formed with a multiplicity of relatively small bores orperforations l8 (e.g., ofa diameter of 300 microns) while the walls 19may be provided with similar bores or apertures 20 to permit escape ofgaseous reaction products.

A gaseous stream is introduced into the shell 16 via the support 12which is connected to a source 21 of a pyrolytically decomposable gascomponent, especially methane, acetylene, benzene, or a chloromethylsilane, and to a source of an inert carrier gas (e.g., argon, helium ornitrogen) as represented by the arrow 22, and constitutes a gasdistributor 23 communicating with the apertures 18. The carrier gas andthe pyrolytically decomposable component flow upwardly (arrows 24).

As the coil 10 is lowered by the leadscrew 14, it heats the coatedparticles 25 filled into the shell and pyrolytically decomposes therising gas stream to deposit a decomposition product 26 as the bondingagent or bridge between the coated particles 25.

The pyrolytic-decomposition front moves progressively downwardly (arrow27) as the coil is lowered. The mass of coated particles and bridgingparticles behind the pyrolysis front and represented at 28 undergoesfusion to produce a coherent mass of 10 to 25 percent porosity,preferably 15 to 20 percent porosity inclusive. In the region of thepyrolysis front, gaseous reaction products emerge through the walls ofthe graphite shell as shown at 29. The body 12, forming a nozzle or tubesupplying the gas to the reactor element 16, is also composed ofgraphite. To permit the assembly 10, 11 to move downwardly at a ratesufficiently slow to permit pyrolysis and fusion, the motor 30 of theleadscrew 14 is provided with a speed regulator 31.

In FIG. 2, we have shown the result of the treatment described withreference to FIG. 1 and this Figure thus illustrates a coated particle32 of a diameter of 400 to 1,000 microns (preferably about 600 microns)with a nuclear reactor fuel or fertile core 33 as described in theaforementioned publication. Between the coating 34 of pyrolytic carbon,silicon carbide, etc., and the adjoining particles 32a and 3217, thereare provided interstices 35 traversed by the gas and sustainingpyrolytic decomposition to produce bridges 36 which fuse to the sheathsof the coated particles at 38. When the coatings are a pyrolytic carbon,the pyrolytically decomposable material is preferably a hydrocarbon suchas methane, acetylene or benzene so that the bridge particles 36likewise are pyrolytic carbon. When, however, the sheaths of the coatedparticles are composed of silicon carbide, for example, the gas streamcontains a methylsilane so that the bridging particles may be pyrolyticcarbon and/or silicon carbide.

The elements 16 may be stacked in a nuclear reactor core 40 into which agaseous coolant, (e.g., helium) is forced at 41, the helium penetratingthe perforated walls of the casing and extending to the innermostportions of the porous mass to effect efficient cooling of the reactorelements.

SPECIFIC EXAMPLES EXAMPLE I tion coil is lowered slowly over a period of1 hour from the upper end of the graphite sleeve to the lower endthereof. The sleeve is thus entirely enclosed. Pyrolytic decompositionof methane and precipitation of carbon results in the bonding of themass into a coherent structure containing a network of interconnectedpores or interstices such that the body has a porosity of about 15percent and the flow resistance of the gas across the element isincreased to a level between 0.190 and 0.326 kp/cm EXAMPLE II Using thesame device as in Example I, 600 micron particles, coated with pyrolyticgraphite or silicon carbide are treated with a hydrogen stream saturatedwith trichloromethylsilane at a temperature of 20C. This gas stream ismixed with argon in a ratio of 1:]. Again, a porosity between 15 and 20percent is obtained and the coherent body is formed by bridgingparticles of silicon carbide.

We claim:

1. A fissionable-fuel element for use in a nuclear reactor comprising amass of large particles of a material selected from the group whichconsists of fissionablefuel or fertile material, said large particlesbeing bonded together by non-fissionable small particles depositedpyrolytically in situ within said mass, said large particles each havinga particle size of 400 to 1,000 microns and being composed of a core ofsaid material, encompassed by a sheath selected from the group whichconsists of pyrolytic carbon, beryllia, magnesia, zirconia, alumina,silicon carbide, zirconium carbide and niobium carbide, said smallparticles consisting of a substance selected from the group whichconsists of pyrolytic carbon, silicon carbide and zirconium carbide andsaid small particles having a particle size substantially less than thatof said large particles.

2. The element defined in claim 1, further comprising a porous graphiteshell receiving said mass and said small particles, said large particlesbeing bonded together in situ within said shell.

3. The element defined in claim 2 wherein said shell has perforatedwalls.

2. The element defined in claim 1, further comprising a porous graphiteshell receiving said mass and said small particles, said large particlesbeing bonded together in situ within said shell.
 3. The element definedin claim 2 wherein said shell has perforated walls.